1. Field of the Invention
This invention relates to a two-axis positioning system, such as a XY plotter or XY table, usable in the field of measuring or control instruments, business machines or industrial machinery.
2. Description of the Prior Art
Conventionally, the two-axis positioning system used for positioning or freely moving the control object at or toward the desired position on the two-dimensional plane has often been so constructed that a rotary motor is used as an actuator for a driving mechanism so that the rotary motion of motor is changed into linear motion through a transmission mechanism, such as a belt, a steel wire or a ball screw, to move or position the position control object, for example, a pen block including a recording pen for the XY plotter, on the XY two-axis plane. Especially, in a case of fixing motors for the X axis and Y axis, two transmission mechanism becomes fairly complicated in construction. From the viewpoint of precise positioning or reliability for the two-axis positioning system, it is of course preferable to use the linear motor as the actuator, instead of the aforesaid transmission mechanism which converts the rotary motion into the linear motion.
FIG. 1 is a schematic perspective view of the conventional XY plotter which uses the drive mechanism of a linear pulse motor of open loop and simple in mechanism and drives the pen block directly in the two directions of the axes X and Y. In FIG. 1, reference numerals 1 and 2 designate a guiding stator and a X-axis movable element at a first X-axis linear pulse motor, the X-axis movable element 2 moving linearly along the guiding stator 1 in the direction of the X-axis (in the drawing, the direction of the arrow X); 3 and 4 designate a guiding stator and an X-axis movable element of a second X-axis linear pulse motor, the X-axis movable element 4 moving linearly along the guiding stator 3 in the direction of the axis X; 5 designates a guiding stator at a Y-axis linear pulse motor, which is mounted at both ends to the X-axis movable elements 2 and 4 moves together therewith in the direction of the axis X, 6 designates a Y-axis movable element at the Y-axis linear pulse motor, which moves linearly along the guiding stator 5 in the direction of the axis Y (in the drawing, the direction of the arrow Y) perpendicular to the X-axis; 7 designates a pen block mounted to the Y-axis movable element 6 and serving as the position control object; 8 designates a flat bed; 9 designates a recording paper; and 10 designates a frame supporting the guiding stators 1 and 3 at the first and second X-axis linear pulse motors and the flat bed 8. In addition, the linear pulse motor, when given one pulse, linearly moves the movable element only in a unit distance, the moving speed depending upon the number of input pulses within a period of unit time. The linear pulse motor, however, is of open-loop control system, whereby when intended to be abruptly accelerated or decelerated, it is liable to cause the pull out because of not completely following the input pulse, and also is very easy to do so during the high speed driving. Hence, the motor speed is limited in a fixed range to have been difficult to perform the high speed response and high speed operation. Furthermore, when one of two X-axis movable elements 2 and 4 at the XY plotter in FIG. 1 is subjected to an external force, the movable elements 2 and 4, which are essentially to operate in synchronism with each other, may cause the step out to lead to a shift in position. Once such condition occurs, the movable elements 2 and 4 connected by the guiding stator 5 cannot be restored in the original synchronism, thereby being twisted to be not easily movable. The twist, if intense, may jam the guiding stator 5. Thus, the system using the linear pulse motors have various defects.